With the release of the “First Report and Order,” Feb. 14, 2002, by the Federal Communications Commission (FCC), interest in ultra wide bandwidth (UWB) communication systems has increased. The IEEE 802.15 standards organization, which is responsible for personal area networks (PAN), has established a task group, TG3a, to standardize a high-data-rate physical layer based on UWB. One of the most important requirements is compliance with the FCC spectral mask for indoor channels shown in FIG. 1. The spectral mask shows power limits as a function of frequency.
In addition to this requirement, it is also required by IEEE 802.15 that UWB systems do not interfere with existing wireless systems, such as 802.15.1 (Bluetooth), 802.15.3 (personal area networks), 802.15.4 (Zigbee) and 802.11a and 802.11b (wireless local area networks).
Furthermore, UWB systems should also be resilient against interference from these existing wireless systems. In addition, UWB systems should also resist interference from home appliances, such as microwave ovens, and other narrowband interferers.
All these requirements impose additional constraints on the spectrum shaping. Minimizing interference, in both receivers and transmitters, requires placing minima on the poser in the transmission spectrum. Rigorously speaking, the spectral mask demands very low power in certain frequency bands, as well as matching filters. A null in the transmit spectrum also implies a null in the transfer function of the matching filter, and thus interference suppression at this frequency.
While some interference, e.g., microwave radiation, is at fixed frequencies, other interference is at variable frequencies, e.g., the different bands of 802.11a, or at frequencies that cannot be predicted a priori. Thus, it is necessary to be able to shape the spectrum adaptively.
For impulse radio signals, pulse-position modulation, (PPM) and pulse amplitude modulation (PAM) are the most popular signaling techniques. These techniques are combined with time hopping (TH) for multiple-access format. In TH, each symbol is represented by a series of “basis pulses.” The location or delay of the pulses is determined by a time-hopping code. The time-hopping code is generated as a pseudo-random sequence.
In general, the spectrum with random code generation can be quite different from the FCC mask. As a result, in order to be compliant with FCC regulation, with a scaling of transmission power, the maximum allowed transmission power can be significantly restricted. Therefore, any signal shaping method that uses a basic, single short pulse must explicitly take the FCC mask into consideration.
Under certain conditions the spectrum of the transmit signal becomes identical to the spectrum of the basis pulse. Therefore, there is a need to modify the spectrum of the basis pulse to fit the requirements of both the FCC mask and industry standards.